The online Catholic Encyclopedia makes this point regarding the soul:
"the rational soul, which is one with the sensitive and vegetative principle, is the form of the body. This was defined as of faith by the Council of Vienne of 1311; the soul is a substance, but an incomplete substance, i.e. it has a natural aptitude and exigency for existence in the body, in conjunction with which it makes up the substantial unity of human nature."
I emphasize that the Church believes the soul is a substance, but not a complete one, which is contrary to the Cartesian conception of the soul. Rene Descartes (although he was a devout Catholic) posited that the soul is a complete but dependent substance: it is a res cogitans which is nonextended spatially and able to survive the body's death in virtue of its latent immortality.
However, the language of "form" used above emanates from Aristotelian philosophy--specifically, the ancient Philosopher's thinking on metaphysics.